In recent years, emerging fields such as new energy vehicles, energy storage, communications, and data centers have developed rapidly, which has greatly promoted the development of large-capacity lithium-ion batteries. Various fields have put forward higher requirements for the energy density of lithium-ion batteries.

The active energy storage material of lithium-ion batteries is a positive and negative electrode material. The way to increase the energy density for the positive electrode is to increase the discharge voltage and discharge capacity. For negative electrode materials, it is high capacity and low average lithium removal voltage.

In the third-generation lithium-ion batteries with the main development goal of improving energy density, the positive and negative electrode materials are in the stage of upgrading and upgrading. In the future, further increasing the energy density will move towards the development of batteries with lithium metal negative electrodes.

What is Battery Energy Density?

Energy density is the measure of how much energy a battery contains in proportion to its weight. This measurement is typically presented in Watt-hours per kilogram (Wh/kg). A watt-hour is a measure of electrical energy that is equivalent to the consumption of one watt for one hour.

Power density is the measure of how quickly the energy can be delivered, rather than how much stored energy is available. Energy density is often confused with power density, so it is important to understand the distinction between the two.

Energy density refers to the amount of energy stored in a certain unit of space or mass of matter. The energy density of a battery is the electrical energy released by the average unit volume or mass of the battery.

The energy density of a battery is generally divided into two dimensions: weight energy density and volume energy density.

How to calculate the energy density of lithium batteries?

Energy density (Wh/L) = battery capacity ×discharge platform voltage/ volume the basic unit is Wh/L

Battery weight Energy density = battery capacity × discharge platform/ weight the basic unit is Wh/kg

The platform voltage of iron batteries: 3.2V; the platform voltage of ternary lithium batteries is generally 3.7V.

Cylindrical volume=πr2×h

Prismatic or other volume = length×width ×height

The greater the energy density of the battery, the more power is stored per unit volume or weight.
Do you know energy density of these rechargeable batteries?

According to the chart ablve, we can easily know that the lithium cell can reach the highest energy density. That is the reason why lithium batteries are widely used all over the world and it can be used in a very wide rang aspects.

So,

What exactly limits the energy density of lithium batteries?

The chemical system behind the battery is the main reason.

Generally speaking, the four parts of a lithium battery are very critical: the positive electrode, the negative electrode, the electrolyte, and the diaphragm. The positive and negative poles are the places where chemical reactions occur, which are equivalent to the second veins of the governor, and their important status can be seen.

We all know that the energy density of a battery pack system with ternary lithium as the positive electrode is higher than that of a battery pack system with lithium iron phosphate as the positive electrode. Why is this?

Most of the existing lithium-ion battery anode materials are mainly graphite, with a theoretical gram capacity of 372mAh/g of graphite. The theoretical gram capacity of the cathode material lithium iron phosphate is only 160mAh/g, while the ternary material nickel-cobalt-manganese (NCM) is about 200mAh/G. According to the barrel theory, the level of the water level is determined by the shortest part of the barrel, and the lower limit of the energy density of lithium-ion batteries depends on the cathode material. The voltage platform of lithium iron phosphate is 3.2V, and the ternary index is 3.7V. Compared with the two phases, the energy density is high and the difference is 16%.

Of course, in addition to the chemical system, the level of production technology such as compaction density and foil thickness will also affect the energy density. Generally speaking, the greater the compaction density, the higher the capacity of the battery in a limited space, so the compaction density of the main material is also regarded as one of the reference indicators of the battery energy density.

In the fourth episode of “Great Heavy Weapon II”, the Ningde Era adopted 6 micron copper foil, using advanced technology to increase the energy density.

How to raise Energy Density of Lithium Battery?

The adoption of the new material system, the fine-tuning of the lithium battery structure, and the improvement of manufacturing capabilities are the three stages for R&D engineers to “dance with long sleeves”. Below, we will explain from the two dimensions of monomer and system.

——Monomer energy density, mainly relying on breakthroughs in the chemical system

01 Increase Battery size

Battery manufacturers can achieve the effect of power expansion by increasing the size of the original battery. The example we are most familiar with is: Tesla, a well-known electric vehicle company that took the lead in using Panasonic 18650 batteries, will be replaced with a new 21700 battery.

However, the “fatness” or “lengthening” of the battery is only a symptom, not a cure. The way to draw the bottom of the kettle is to find the key technology to improve the energy density from the positive and negative electrode materials that make up the battery cell and the composition of the electrolyte.

02 Change Chemical system

As mentioned earlier, the energy density of the battery is subject to the positive and negative electrodes of the battery. Since the current energy density of the anode material is much larger than that of the cathode, it is necessary to continuously upgrade the cathode material to increase the energy density.

High nickel positive electrode

Ternary materials generally refer to the large family of lithium nickel-cobalt-manganese oxide oxides. We can change the performance of the battery by changing the ratio of nickel, cobalt, and manganese.

As can be seen from several typical ternary materials in Figure 5, the proportion of nickel is getting higher and higher, and the proportion of cobalt is getting lower and lower. The higher the content of nickel, the higher the specific capacity of the cell. In addition, due to the scarcity of cobalt resources, increasing the proportion of nickel will reduce the use of cobalt.

Silicon carbon negative electrode

The specific capacity of silicon-based anode material can reach 4200mAh/g, which is much higher than the theoretical specific capacity of graphite anode 372mAh/g, so it has become a powerful substitute for graphite anode.

At present, the use of silicon-carbon composite materials to increase the energy density of batteries is one of the development directions of lithium-ion battery anode materials recognized by the industry. The Model3 released by Tesla uses a silicon-carbon negative electrode.

In the future, if you want to go one step further-breaking through the threshold of 350Wh/kg for single cells, peers in the industry may need to focus on lithium metal negative electrode battery systems, but this also means the entire battery production process changes and refinement.

03 System energy density: improve the grouping efficiency of battery packs

The grouping of battery packs tests the ability of the battery “siege lions“ to line up single batteries and modules. It is necessary to take safety as the premise and maximize the use of every inch of space.

There are mainly the following ways to “slim down” the battery pack.

Optimize the arrangement structure

In terms of dimensions, the internal layout of the system can be optimized to make the internal parts of the battery pack more compact and efficient.

Topology optimization

Through simulation and calculation, we realize the weight reduction design under the premise of ensuring the rigidity and structural reliability. Through this technology, topology optimization and morphology optimization can be realized, and ultimately help to realize the lightweight of the battery box.

Material selection

We can choose low-density materials, such as the upper cover of the battery pack, which has gradually changed from the traditional sheet metal upper cover to the composite upper cover, which can reduce weight by about 35%. For the lower box of the battery pack, it has gradually changed from the traditional sheet metal scheme to the aluminum profile scheme, reducing weight by about 40%, and the lightweight effect is obvious.

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Q&A

1.Which battery has highest energy density?

Compared to the other high-quality rechargeable battery technologies (nickel-cadmium or nickel-metal-hydride), Li-ion batteries have a number of advantages. They have one of the highest energy densities of any battery technology today (100-265 Wh/kg or 250-670 Wh/L).

2. Why Do You Need a Battery with High Energy Density?

To better understand lithium-ion batteries, you should understand why a high energy density is a desirable trait in a battery. A battery with high energy density has a longer battery run time in relation to the battery size. Alternately, a battery with high energy density can deliver the same amount of energy, but in a smaller footprint compared to a battery with lower energy density. This greatly expands the possibilities for battery applications.

In factory or warehouse settings, forklift batteries can weigh thousands of pounds. A lightweight battery for forklifts offers some advantages in safety and handling.

If the energy density of a battery is too high, it could present a safety issue. When there’s more active material packed into a cell, it increases the risk of a thermal event.

3.What is the monomer energy density?

The energy density of a battery often points to two different concepts, one is the energy density of a single cell and the other is the energy density of the battery system.

A battery cell is the smallest unit of a battery system. M batteries form a module, and N modules form a battery pack. This is the basic structure of a vehicle power battery.

The energy density of a single cell, as the name suggests, is the energy density at the level of a single cell.

4.What is the energy density of the system?

The system energy density refers to the weight or volume of the entire battery system after the monomer combination is completed than the entire battery system. Because the battery system contains a battery management system, a thermal management system, a high and low voltage circuit, etc., which occupy part of the weight and internal space of the battery system, the energy density of the battery system is lower than that of the monomer.

System energy density = battery system power/battery system weight or battery system volume.

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